In many power supply applications, such as gas discharging light bulb, high-voltage DC bus of inverter in un-interrupt power supply and wide-frequency traveling wave tube amplifier etc, high-voltage DC power supplies are needed where rechargeable batteries provide power source; also, clean energy sources, such as solar power, wind power and fuel-cell battery, have relative low output voltages of DC power sources. It is necessary to have high-efficiency high voltage boost ratio DC-DC converter as front-end conversion mechanism. The present invention comprises a high voltage boost ratio converter, which utilizes bi-directional coupling induction magnetic energy transfer, such that: (1) converting traditional rechargeable battery and clean energy supply system into high-voltage DC power supply systems and substantially increasing energy efficiency and stability of power source; (2) after converting AC to DC boosting the voltage to thousand volt, or adjusting DC voltage and providing back-end power supply and improving quality of power supply. Although the present invention involves a wide range of technology fields, such as, electrical electronics, DC/DC conversion technology and energy technology, mainly deals with bi-directional magnetic energy transfer using a coupling inductor and rectifying techniques to improve current and voltage stresses exerted on components of boost converter, which affects full potential and conversion efficiency.
Traditional boost converter circuit, as shown in FIG. 1(a), utilizes duty cycle of modulating switch to boost output voltage potential. Since the voltage across the switch will equal output voltage when the switch is turned off, semi-conductor power switch with high voltage withstand has to be used; if MOSFET is used it will introduce high conduction loss because it has large conduction resistance (RDS(ON)). Furthermore, due to reverse-recovery problem in the output of diode of traditional boost converter, at the moment when switch is turned on the diode has to use spike current to establish reversed bias voltage such that this current pass through the switch causing switching loss and lower efficiency. But, since it is simple in structure and has low cost, it is been used widely, such as Power Factor Correction (PFC), when high voltage boost rate and efficiency are not concerned.
Second kind of commonly used configuration is transformer; the major advantage is separation between high and low voltage circuits. The transformer in DC-DC converter normally decreases voltage, which prevents components at low voltage side from damaged by high voltage side leakage current. Still, problems, such as, balance control of magnetic excited current and leakage current control, have to be addressed. Moreover, when used in boosting voltage, transformer configuration has many drawbacks, i.e., highest gain ratio equals only to the turn ratio of winding, output rectify diode is under twice stress of output voltage, making snubber necessary.
To boost converter, if circuit is controlled by low voltage side, it is not necessary to have circuit separation since semi-conductor switch with low specified voltage withstand is used and control circuit can control system's voltage, plus controlling switch utilizes voltage clamping technique. Non-separation boost configuration is developed where commonly used coupling inductor boost circuit, as shown in FIG. 2(b), has a character of high boost ratio of flyback design. Due to the coupling inductor is a non-separation boost configuration and primary circuit can assist in boosting voltage, it has higher boost ratio and output power than flyback circuit. However, when switch is turned off, spike voltage produced by leakage induction could damage the switch if no snubber is installed to absorb its energy, which decreases circuit efficiency.
To improve above mentioned traditional boost converter, high efficiency boost converter technologies are recommend by many experts, which can be categorized into following four types:
I. Soft Switching Technique
In reference article “A single-switch continuous-conduction-mode boost converter with reduced reverse-recovery and switching losses,” by Lu et al., IEEE Transactions on Industrial Electronics, vol. 50, pp. 767–776, 2003, taking advantage of resonance between leakage of the coupling inductor and parasitic capacitance (or output capacitance), the switch is turned on when the resonant voltage is at lowest point, such avoiding diode's reverse-recovery current and reducing switching loss, plus it's simple switch configuration and efficiency will reached above 97% with low load. The drawbacks are: (1) the switch still has to endure stresses from both high and low side of voltage and current; (2) low utilization of switch's full capacity, such as TO-247 switch can output only 200 W, losing efficiency under heavy load; (3) high induction current ripples and conduction loss; (4) low boosting ratio, only 50% higher than input voltage; (5) frequency conversion making circuit complex and limiting effectiveness of soft switching under heavy load. Normal resonance circuit can be easily affected by the load and parameters of inductor and capacitor, and has large switching current ripples causing extra conduction loss. In reference article “An improved family of ZVS-PWM active-clamping DC-to-DC converters,” by Duarte et al., IEEE Transactions on Power Electronics, vol. 17, pp. 1–7, 2002, its output reaches 1.6 kW and has higher conversion efficiency than previous one; but an auxiliary switch is necessary, making control circuit more complex. Soft switching has become a key in high efficiency conversion technology since it has low conduction current and output voltage difference between 400V and 300V is not quite large. In general, non-separation converter with high input voltage and low boosting ratio has low conduction loss when diode's reverse-recovery current is dealt with properly, since short switch conducting time means only energy difference between output and input being provided by the switch; in theory conversion ratio can be augmented substantially. Essentially, when dealing with soft switching the most important is, when the switch is turned on, the short current loss of switch's parasite capacitor; if does not count diode's reverse-recovery current part, switch MOSFET major part of switching loss equals to 0.5fsCossVDS2, where fs is switching frequency, νDS is switch voltage and Coss is switch parasite capacitor; if both sides' voltage is lower than 50V before the switch is turned on, the percentage of switching loss in overall loss will decrease; therefore, effectiveness of soft switching to voltage manipulation is limited as to improving conversion efficiency.
II. Transformer Boost
In reference article “An improved boost PWM soft-single-switched converter with low voltage and current stresses,” by Silva et al, IEEE Transactions on Industrial Electronics, vol. 48, pp. 1174–1179, 2001, by combing transformer and soft switching technique, its efficiency can reached 97.5% while boosting ratio is less than 3 times and far less than the turn ratio of winding. The stress on the switch equals to output voltage such that low voltage low conduction loss semi-conductor power switch can not be used since the transformer did not function as a separation device.
III. Coupling Inductance Configuration
In reference article “High-efficiency, high step-up DC-DC converters,” by Zhao et al., IEEE Transactions on Power Electronics, vol. 18, pp. 65–73, 2003, successfully solved leakage induction energy problem and achieved goal of switch voltage clamping. Clamping capacitor is used to absorb large transient current at low voltage side and helps boosting voltage gain. Also, stress on the switch is lower than output voltage and it has highest boosting rate among aforementioned techniques; fair conversion efficiency is achieved even under the maximum power output condition, and high efficiency and boosting ratio converter becomes possible. Later, in sequent article “Novel high-efficiency step-up converter,” by Tseng et al., IEEE Proceedings Electric Power Applications, vol. 151, pp. 182–190, 2004, when switch is turned on, stress on diode at high voltage side equals to reverse bias VO+nVIN (VO and VIN are output voltage, n is turn ratio of winding), making it necessary to install snubber to eliminate spike voltage caused by leakage induction, which becomes more apparent when high output voltage and high turn ratio of winding are involved. Even though output capacitor is adjusted to secondary return route of high voltage, effectively reducing reverse bias on diode, snubber is still necessary.
IV. Secondary Side Multiple Series Boosting
In reference article “Isolated DC-DC converters with high-output voltage for TWTA telecommunication satellite applications,” by Barbi et al., IEEE Transactions on Power Electronics, vol. 18, pp. 975–984, 2003, single or two step configuration, soft switching and transformer boosting are all combined to obtain high voltage gain. After the secondary of transformer is rectified, multiple winding are connected in series to attain 3.2 kV high voltage output, which primarily used as power source for satellite telecommunication, similar to Tseng et al design. Because of the character of soft switching, which effectively resolved high voltage side reverse-recovery current on the diode, conversion efficiency is very high; when input voltage is at 26V–44V, supply 150 W to a load, lowest efficiency is 94.1% and it is a classical among boosting technologies. Detail analysis reveals that 3.2 kV is achieved by using multiple winding voltage connected in series; if single winding used the highest output voltage is only 750V. Major components include four switches, three inductors and one transformer. The highest voltage measured on auxiliary switch is 150V while a 250V–23 A switch is chosen; the highest voltage measured on main switch is 120V while a 200V–100 A switch is chosen. All the switches are TO-247 with output power only 150 W, where their full capacity is not utilized, since efficiency is the primary concern in satellite telecommunication.
Referring to the all above mentioned technologies and other coupling inductor configurations, voltage waveforms on switch are as shown in FIG. 15 referred to Lu et al or FIG. 19 referred to Tseng et al. measured on MOSFET switch, where spike voltage occurs at the turn-off instant and is 50% higher than normal voltage across switch forcing higher voltage withstand switch to be used and may be higher than output voltage. To MOSFET RDS(ON) increases proportionally far higher than voltage increase; generally speaking, conduction loss for MOSFET is proportional to the square of the output current and conduction loss for high voltage MOSFET under heavy load will be higher than that for IGBT semi-conductor power switch, which is where attention being paid by researchers. The switch spike voltage presented in Lu et al. and Tseng et al. is induced by instant current change from circuit and components' internal induction current when primary coupling inductor is shut off. To solve the problem a snubber is connected to the switch in parallel and shorter the path the better it is; the path must have low skin effect and mutual resonance as to effectively using lower voltage, low conduction loss switch; for high efficiency high boost ratio equipment, voltage clamping technique is, such, much more important than soft switching technique. Additionally, although above-mentioned coupling circuit has eliminated the impact of leakage induction, they did not further resolve the problem of voltage clamping at high voltage side of diode. Also, the iron core is not fully utilized since secondary winding has only single direction current.
In summary, the drawbacks of above boost converter technologies are: (1) resonance circuit applies to the configuration of high input voltage; (2) switch capacity is not fully utilized; (3) voltage clamping can't be achieved on both high and low voltage sides at the same time; (4) magnetic excited current and induction current of transformer are not fully utilized; (5) conversion rate can't be increased substantially; (6) there is not any configurations can achieve high efficiency and high boosting rate at the same time; (7) configuration and control are complex.